The Influence of Latent Heat and Topography on the Evolution of Midlatitude Tropical-Like

Cyclones

Hurricane Catarina

Chris Simmons

Thesis Advisor: Dr. Redina Herman

Geography 405

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Contents Abstract 2 1 Introduction 3 1.1 Past Research 5 1.2 Statement of Purpose 9 2 Methodology 9 3 Open Ocean Storms 12

3.1 Hurricane Catarina 12 3.1.3 Early Mature Stage 13 3.1.2 Mature Stage 19 3.1.3 Late Mature Stage 22

3.2 Hurricane Vince 29 3.2.1 Secondary Circulation 31 3.2.2 Vince’s Kicker Trough 33 4 Topography-Enhanced Storms 35 4.1 Early-Mid October, 1996 Medcane 37 4.1.1 200 mb Characteristics 37 4.1.2 Tropopause-Level Pressure 41

4.2 Early October 1996, 1983 Medcanes 44 4.3 1995 Medcane 46

5 Discussion 5.1 Limitations of Subtropical Jet 48 5.2 Limitations of Polar Jet 52 5.3 Shelter from Kickers 55 6 Conclusions 57

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The Influence of Latent Heat and Topography on the Evolution of Midlatitude Tropical-Like Cyclones Abstract:

Located between latent-heat driven tropical storms and largely sensible heat-driven

convective “arctic hurricanes,” midlatitude tropical-like cyclones are increasingly being studied

due to their formation near regions of high population density. Recent research has concluded

that latent heat fluxes from the surface dominate these storms’ convective growth, but few have

focused extensively on the motion of this energy once it reaches the upper atmosphere and how

the storm modifies its environment through the introduction of latent heat into their parent cold

upper level lows. This study thus seeks to analyze the nature of latent heat above midlatitude

tropical-like cyclones, with particular emphasis on the effects of orography on the flow of this

heat. In order to do this, two primary cases are distinguished—midlatitude tropical-like cyclones

that form over the open ocean away from orography (Hurricanes Catarina in 2004 and Vince in

2005) and tropical-like cyclones that develop in regions where upper-level dynamics are strongly

affected by the presence of mountains (medcanes and the Kanto typhoon in 2003). For each case,

National Centers for Environmental Prediction (NCEP) reanalysis data, backed by available

remote sensing observations, is used to track the synoptic scale blocking pattern and illustrate the

role that latent heat plays in the upper atmosphere above the storm. This research then discusses

the importance of this latent heat flow in forecasting the development and strength of midlatitude

tropical-like cyclones occurring in both topographical environments and nonorographic open

ocean settings.

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1 Introduction

Midlatitude tropical-like cyclones are rare, recently-discovered hurricane-like phenomena

that form well to the north of the regime normally associated with tropical and subtropical

cyclones. They tend to develop underneath or near the center of a cold airmass moving over

warm waters and are most frequently observed over the Mediterranean sea (where they are called

Mediterranean hurricanes or just simply ‘medcanes’) and off the east coasts of major continents.

However, unlike most tropical hurricanes, they tend to stay small (with diameters of only 100-

200 km) and have limited wind speeds that struggle to reach Category 1 force and rarely obtain

Category 2 strengths. In addition, they tend to develop very rapidly and maintain eyes, eyewalls,

and spiral bands long before reaching hurricane intensity winds, also unlike most tropical

cyclones. Some meteorologists in the past have associated medcanes and similar midlatitude

tropical-like storms with polar lows because both types of storms form in a similar environment--

within deep troughs and cold polar airmasses--but this conjecture has been largely discounted

due to medcanes’ strong dependence on the latent heat flux, noted by Lagouvardos et al. (1999)

in the 1995 cyclone, Meneguzzo et al. (2001) in the 1997 medcane, and Sousounis et al. (2001)

in Hurricane Huron, whereas Twitchell et al. (1989) indicates that convective polar lows rely

more heavily on sensible heat fluxes. Midlatitude latent heat-driven storms also possess many

similarities to subtropical cyclones, including an initial baroclinic structure, although according

to Bosart et al. (2003), subtropical storms rely on warmer sea surface temperatures near 26°C

and often fail to develop when their parent troughs move over colder water. By contrast,

midlatitude tropical-like cyclones can form in waters well below the standard 26°C threshold,

sometimes developing over seas as cool as 13°C (Meneguzzo et al., 2001). In addition, Bosart et

al. (2003) also indicates that many subtropical storms must modify their environment through

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latent heat fluxes to reduce shear before becoming closed tropical cyclones, whereas most

medcanes tend to develop within naturally low shear environments (the centre of an upper level

low) and evolve from short-lived baroclinic spinups that lack the kind of strong shear and

temperature contrasts that Bosart et al. (2003) discovers in many incipient subtropical cyclones

in his study.

Therefore, midlatitude tropical-like systems appear to be in a class of their own, a hybrid

storm type somewhere along the continuum of convective, sea-driven cyclones between

subtropical storms and polar lows. These storms form on rare occasions across the world, and

they usually have been considered separate phenomena (until recently), with some receiving the

title of ‘hurricane’ and others not, depending on their location. However, meteorological

researchers are increasingly beginning to recognize the similarities between different tropical

cyclones systems that form poleward of 30 degrees latitude, such as medcanes, east coast

Australia hybrid systems, South Atlantic Hurricane Catarina, and North Atlantic Hurricane

Vince. For example, in his presentation to the International Conference on Hurricane Catarina,

Dr. Greg Holland provided a compelling argument linking Catarina and hybrid midlatitude

tropical-like storms off the east coasts of Australia and the United States, and Manuel Gan from

the Instituto Nacional Pesquisas Espaciais in Brazil provided arguments connecting the

similarities between Hurricane Catarina and tropical like storms. In addition, Kumabe et al.

(2005) specifically link the typhoon-like Kanto storm with the very similar medcane

phenomenon. Thus, midlatitude tropical-like cyclones are increasingly becoming a recognized

phenomenon in their own right, connected by their common characteristics as small, centralized,

eyed storms developing underneath upper level lows and possessing winds that rarely exceed 75

mph.

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1.1 Past Research

Research associated with midlatitude tropical-like cyclones are limited to a handful of

case studies, most of which focus on the medcane phenomenon. The best recognized research on

these cyclones center around the 1982, 1983, 1996, early and early-mid October 1996, and 1997

medcanes, and this paper is also limited to analyzing these particular Mediterranean storms. The

lack of data, and sometimes the presence of erroneous data, has provided serious limitations in

most of the previous literature surrounding these systems. For example, incorrect sounding

readings is considered the primary reason for the failure of two separate long-term model runs

provided by Kuo et al. (2002) and Reed et al. (2001) in their respective analyses of the 1982

medcane. Other authors, such as Pytharoulis et al. (2000) analyzing the 1995 medcane, also note

the need for better data coverage to form a more complete picture of the cyclone’s environment.

For the purposes of analyzing the flow of latent heat, extensive spatial and temporal atmospheric

sounding coverage, high resolution satellite-derived sounder wind data, AMSU brightness

sounder profiles, and hurricane hunter data will be needed in future medcanes and similar

cyclones in order to provide a better understanding of the upper level dynamics and

thermodynamics of midlatitude tropical-like storms.

Most medcane studies primarily focus on these storms’ synoptic evolution, the nature of

lower level sea surface fluxes within these systems, and the dynamic transfer of momentum from

the parent jet to the lower levels during the initial formation stages of these cyclones. Although

none of the prominent medcane papers, including Reale et al. (2001), Pytharoulis et al. (2000),

Lagouvardos et al. (1999), and Meneguzzo (2001), discuss the upper troposphere in close detail,

clues from their respective model runs are helpful for the purposes of this discussion of latent

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heat. For example, Pytharoulis et al. (2000) notes that strong upper level divergence (associated

with latent heat being transported out of the storm centre) is not observed over the 1995

medcane, with only a very weak anticyclonic circulation observed above the storm during its late

mature stage. In addition, Reale et al. (2001) indicates that no clear anticyclonic divergence is

seen at the upper levels in his study of the October 1996 medcanes. Thus, clearly the lack of a

strong anticyclone or rapid divergence from the centre of these storms provides crucial

information about the nature of medcanes forming within cut off lows and lends support to the

proposition presented in this paper that these convective storms tend to evolve according to the

depth of the upper level low and dissipate after latent heat has completely filled in its associated

synoptic cut off.

Despite the limited information surrounding medcanes, the formation of Hurricane

Catarina in 2004, which quickly became famous as the first hurricane in the South Atlantic,

promoted a flurry of new research in the field of midlatitude tropical-like cyclones and provided

an opportunity to analyze a strong medcane-like storm outside of the Mediterranean basin with

the addition of more remote sensing data depicting conditions at the upper troposphere. Of the

recent studies surrounding this cyclone, McTaggert-Cowan et al. (2005) perhaps provides the

most extensive explanation available concerning the motion of latent heat by providing particular

emphasis on the dipole of the Rex Block (ridge in polar jet aligned with trough in subtropical jet)

associated with the cyclone and suggesting that the polar jet ridge was built by outflow from the

upper level latent heat outflow of the storm. McTaggert-Cowan et al. (2005) offers detailed

drawings of the theoretical evolution of upper level flow; however, the study’s focus is more

closely related to the midlevel (500 mb) flow pattern provided by dipole scenario consisting of a

low pressure circulation located directly north of a high pressure circulation. Therefore, this

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publication presents little on the upper troposphere and fails to provide figural evidence that

latent heat actually served to build a ridge in the adjacent polar front. A clearer picture of the

upper troposphere is, however, available from satellite-derived sounder wind images taken every

24 hours, and these are discussed more extensively in this paper to illustrate the flow of upper

level latent heat around the storm's parent trough.

In their analysis of Hurricane Huron, Sousounis et al. (2001) also provides another

important discussion of latent heat release within the upper level trough that proves crucial for

this paper. They argue that as the latent heat fluxes filled into the upper level low, causing

pressure heights to rise and decreasing the vorticity and thus the vertical motion within the

cyclone. As Sousounis et al. (2001) implies, hurricanes that form in broad, upper tropospheric

ridges tend to increase pressure heights around the storm and amplify the ridge in a nearby jet

through the introduction of warm, latent heat into the outward-bowing jet. Similarly, convective

storms introducing large quantities of latent heat in a region of lower upper level pressure would

have the same filling effect, but instead of amplifying a ridge it serves to fill in the pressure

heights in its parent trough, which causes the trough to weaken and dissipate. And unlike the

ridge environment provided over most hurricanes, the upper level lows over midlatitude tropical-

like cyclones also provides an environment that contains the latent heat that is released into the

upper atmosphere rather than aiding in its spread.

Correspondingly, while this heat may diverge within the upper level low, mean cyclonic

flow surrounding it generally limits the spread of such latent heat unless it is able to fill in the

heights above the storm to the point that it forms a mesoscale high pressure anomaly. The

presence of this kind of warm pertubation within a broader scale upper tropospheric low could

create outward diverging winds from the centre of the storm that subsequently flow cyclonically

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around a low pressure minimum now shifted closer to the trough by rising heights over the

original low pressure centre. However, if the trough is weak or associated with a substantially cut

off low (an isolated cold pool of air), heights simply rise and the upper level system appears to

fill in as demonstrated by Sousounis et al. (2001), and the accumulation of latent heat at the

upper levels overwhelms the parent storm and causes increased subsidence that brings about the

cyclone's dissolution. Thus, the strength of the Hurricane Huron seems to be closely related not

only to the depth of the parent trough (which determines the coldness of the air within it that

promotes surface fluxes) but also to the amount of latent heat accumulated in the trough over the

course of the storm's lifecycle. This paper takes this idea further by analyzing which midlatitude

tropical-like cyclones tend to be associated with the simple filling of heights above the parent

trough (like Hurricane Huron) and which manage to maintain upper level divergence that

disperses heat more efficiently within the parent trough or toward the jet.

In general, when a bubble of warm air accumulates above a large-scale convective storm,

tropopause heights increase, producing a tropopause-level protrusion into the stratosphere that

can be resolved by NCEP and other data analysis methods. An MTP observations study for

Hurricane Emily provides a close analysis of standard tropical cyclones at the tropopause level,

and it reveals that the tropopause level increases in height above the strongest convection in the

eye wall and decreases in height above the rest of the storm (MTP). Because most latent heat

rises above the eyewall and diverges away from the storm center, the highest tropopause heights

(warmest temperatures) are located immediately above the eye wall (and especially over the

region of strongest convection within the eyewall) and dip down in the eye, where the strong

subsidence associated with inward converging winds pulls down on the tropopause (Krichak et

al., 2005).

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1.2 Statement of Purpose

While this past research provides important clues about the nature of latent heat once it

enters the upper levels, the authors of these studies do not exclusively or extensively describe or

depict (using figures) the build up and flow of latent heat within the systems that they analyze.

Therefore, this study attempts to fill the void in research on the thermodynamic structure of the

upper levels above midlatitude tropical like cyclones and illustrate though both direct and

indirect methods, using available remote sensing and sounding reanalysis data, the nature of

latent heat flow above several storm examples. In addition, this paper focuses on the effects of

topography on the flow of latent heat and dynamic interactions with the jet streams to the north

and south of the storm. The analysis of latent heat above midlatitude cyclonic storms is divided

into two cases: a non-orographic, open-ocean case, in which hurricanes Catarina and Vince are

extensively analyzed, and a orographic case, in which midlatitude tropical-like cyclones

(medcanes and the 2003 typhoon-like Kanto storm near Japan) forming in regions affected by

high topography are the primary focus. The differences in strength between systems evolving in

two cases are compared and subsequently related to the effectiveness of the transport of latent

heat away from the storm centre.

2 Methodology

NCEP reanalysis data provides the primary evidence in this paper concerning the

relationship between latent heat fluxes, the location of circulation anomalies, and topographical

barriers. Each graphic collected from NCEP proved particularly helpful in reproducing the

synoptic-scale blocking patterns associated with medcanes and even Hurricane Catarina as well

as broad changes within the jet pattern, such as rising pressure heights in upper level lows

associated with medcanes. The NCEP pressure height results for the medcane examples were

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compared to upper level maps provided in the papers of Reale et al. (2001), Pytharoulis et al.

(2000), Lagouvardos et al. (1999), and Kumabe et al. (2005), and in each case the NCEP data

closely mirrored the synoptic maps offered in these studies. Similarly, the synoptic scale pattern

associated with Hurricane Catarina also closely reflected similar NCEP reanalysis conducted by

other researchers, such as in Dr. Menezes presentation to the International Conference of

Hurricane Catarina (2004). Thus, due to the close resemblance between NCEP data and real

world observations and model output produced in other studies, NCEP’s estimation of large scale

patterns was considered to be relatively accurate.

However, NCEP reanalysis depends largely on the presence of dense sounding data in

order to provide an accurate depiction of the atmospheric setup for a particular time period, while

processes over regions lacking such data are simply estimated by the model given nearby

information that is available. Thus, when providing information about open ocean storms like

Hurricane Catarina and Vince, NCEP images proved less accurate at higher resolutions due to

greater estimation and fewer details than in regions surrounded by many sounding stations, such

as the western Mediterranean basin. Therefore, for the purposes of this study NCEP graphics

collected from medcanes that passed close to land or made multiple landfalls (such as the 1983

and mid October 1996 medcanes) were considered more reliable and able to depict smaller

changes associated with the subsynoptic cyclone in the larger scale pattern. Correspondingly,

Mediterranean tropical-like storms were analyzed at a higher resolution (15 degrees longitude by

15 degrees latitude in some cases) than those over the open ocean like Catarina, which were

analyzed at a variety of scales before a large 26 degrees latitude by 35 degrees longitude grid

was finally accepted as most reliable. Due to the serious limitations of using a large scale model

such as NCEP, which tends to smooth over small scale anomalies, to demonstrate processes

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associated with subsynoptic-scale storms, references to raw remote sensing data, such as satellite

images, brightness sounder data, and satellite-derived winds, were used when available to

confirm processes seen in NCEP data and to illustrate smaller mesoscale details not visible on

NCEP model estimations.

The use of upper level NCEP temperature and pressure data also requires additional

consideration. The NCEP model determines upper tropospheric temperatures above the 250 mb

level according to the depth of the pressure heights. Lower pressure heights (associated with

warm stratospheric air above the 250 mb level) have warmer temperatures in each medcane

example analyzed, while higher pressure heights appear to be associated with cooler

temperatures (due to the greater elevation of the tropopause and less warm stratospheric air in

this region). Thus, as an upper level low gradually fills, temperatures appear to decrease on a

broad scale within the 200 mb level low pressure height region because the elevation of the

pressure levels and the warmer stratospheric air is increasing. However, NCEP output images do

represent the actual temperature profile at the upper troposphere, and an introduction of warmer

air at a specific pressure level causes temperatures to spike within the region on an associated

NCEP image. So while temperatures on a broad scale may appear to be decreasing in upper level

lows co-located with medcanes (and pressure heights appear to be increasing), upper level

temperatures may be steady or increasing in a smaller region over a convective storm within the

larger scale pattern, and these anomalies are considered important in this paper. Similar to upper

level temperature depictions, tropopause level pressure and temperature data follow the same

pattern, and concentrated, persistent warm anomalies within a broader decreasing temperature or

pressure height region are similarly analyzed as representative of latent heat tendencies.

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3 An Analysis of the Flow of Latent Heat Above Open Ocean Midlatitude Tropical-Like Cyclones: The Divergent Circulation of Latent Heat 3.1 Hurricane Catarina

Despite the lack of accurate and confirmable thermodynamic NCEP profiles associated

with Hurricane Catarina due to the storm’s oceanic evolution, some remote sensing data is

available from various stages of the cyclone’s life. Thus, Catarina is used as a control in this

paper because it provides one of the few examples of midlatitude tropical-like cyclone

development over the open ocean (without topographic influences) where a variety of data is

available. However, there are two important stages in this cyclone’s lifecycle, evolution and

dissipation, when topographic land features do play an important role. Before the development of

the subsynoptic, latent heat-driven storm, the low-altitude coastal mountain range provided

crucial PV enhancement and upper level support (a small shortwave disturbance) for the original

surface baroclinic low that eventually occluded and formed the convective foundation for

Catarina. In addition, at landfall, increased land-induced friction convergence and orographic lift

as the storm moved toward Brazil’s coastal range led to explosive vertical development along

Catarina’s left flank and perhaps even brief convective reintensification of the storm (indicated

by the persistence of the large eye feature after landfall). This process is illustrated in Figures

1(a) and 1(b), sequential IR images of the storm as the eye made landfall and pushed inland.

However, as indicated by the Figure 4 below, the trough environment containing Catarina

remained relatively isolated from both continental influences and orography during the mature

stage of the cyclone over the open ocean.

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Figure 1(a): IR satellite, Catarina before landfall Figure 1(b): IR satellite, Catarina after landfall

3.1.1 Hurricane Catarina’s Formation Stage: Rapid Latent Heat Accumulation Leads to the Premature Development of an Eye

The early stages of Hurricane Catarina are particularly helpful in illustrating the primary

role that latent heat plays in the storm’s development and provides an important example of a

characteristic—the early development of the eye—seen in both Catarina and other midlatitude

tropical-like cyclones. According to the Carnot Engine theory of energy transport within

standard tropical cyclones, when a large amount water vapor condenses and releases latent heat

within a hurricane, most of this energy rises to the top of the troposphere where it is largely

forced to spread out due to the stratospheric inversion above it that often restricts additional

rising motion. Only a relatively small portion of the total outflow converging in the storm center

is forced back down through the eye, while much of this energy is diverged rapidly outward.

Figure 3(a) roughly illustrates this process of Carnot Engine circulation around the storm.

Without this strong upper level divergence, dry latent heat would not be able to circulate out of

the hurricane and the system would be unable to strengthen due to the continued compression of

air molecules being transported to the top of the storm (which then creates a high pressure

environment at the upper levels and strong subsidence that inhibits convection) (Bosart et al.,

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2003). However, satellite-derived winds shown in Figure 2 indicate that, unlike standard tropical

cyclones, the wind flow at the upper levels above Hurricane Catarina during the storm’s

formation stage is cyclonic and associated with the upper level low, not anticyclonic and

divergent as is most often viewed over tropical cyclones (Pytharoulis et al., 2000).

0 = (-1/ρ)*(∆P/∆x) + fc*V - wT*(U/zi) + s*(V*│V│)/R

0 = (-1/ρ)*(∆P/∆y) -fc*U - wT*(V/zi) - s*(U*│U│)/R

Formula Series 1: Boundary-layer gradient wind balance equations, where ρ is density, P is pressure, V is the meridional wind component, U is the zonal wind component, fc is the coriolis parameter, R is radius of curvature, s is the centrifugal force sign coefficient, wT is turbulent transport velocity, zi is the depth of the turbulent layer, and x and y are zonal and meridional distances respectively.

The upper level low environment, however, proves particularly unique to midlatitude

tropical-like cyclones and provides this cyclonic motion. Normally, cyclonic rotation within an

upper level low is not convergent, as turbulent drag is usually negligible in the upper

atmosphere, and the lack of friction, by the Boundary-Layer Gradient Wind Balance equations

(see Formula Series 1), minimizes the convergence seen within upper level lows. However,

Figure 2 indicates that some upper level winds are converging from the outer spirals toward the

center of the cyclone (inward spiraling, especially well seen on wind barbs on the south side of

the storm associated with the strongest convection), with little indication of any major upper-

level divergence of latent heat (but the beginnings of which are seen in slow, outward-spiraling

cyclonic rotation hinted by the yellow wind barbs in Figure 2, starting at the eye of the storm).

The presence of convergence in the upper levels above the storm center seems surprising,

as convective activity associated with the incipient storm had evolved throughout the previous

day, likely increasing the amount of latent heat in the upper atmosphere that would normally

resist such cyclonic flow and lead to rapid divergence. However, the deep upper level low

regime, with substantially lowered pressure heights, prohibited the outward expansion of latent

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heat during the storm’s early stages. In addition, the slightly convergent winds seen in the upper

level low was likely produced by a turbulent drag force, associated with the rapid ascent of latent

heat within the incipient storm clouds themselves, which created a dynamic force balance

slightly more similar to low pressure boundary layer winds. In turn, this latent heat was forced to

accumulate above the system and converge toward the center of the upper level low, creating a

buildup of air molecules immediately above the storm that would have been pushed down

through the center of the storm—seen as the eye. In other words, air forced to converge into the

center of the storm, its motion limited by the low stratospheric lid and air flowing in from all

directions, was pushed down at the center of the upper level low, creating a pronounced region of

subsidence resulting in the development of a strong, well defined eye feature. In addition, the

large vertical scale of the convection served to provide a turbulent drag force and inward

convergence into the vertically-stacked low at all layers, creating a wind flow pattern at the mid

and upper troposphere that is more similar to that at the surface. Thus, while providing strong

subsidence, the creation of boundary layer-like winds also served to reduce vertical directional

wind shear over the storm and provided more favorable conditions for convective tropical

cyclone-like development along with the development of an eye before the storm reached

standard hurricane intensity.

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Figure 2: Upper Level Winds above Hurricane Catarina, early mature stage, 1500 UTC 25 March, 2004

Satellite images associated with Catarina also lend support to this conjecture concerning

the dynamics behind early eye development within midlatitude tropical-like storms. As vertical

transport increased in Hurricane Catarina, an arc of cumulonimbus clouds blossomed around the

southern side of the storm, but the pattern of cumulonimbus development maintained strong

suppression of development in the very center of the system, associated with a constantly clear

eye. Satellite-derived scatterometer winds and other estimates (not shown), however, place the

strength of the storm at tropical storm force (wind speed: 20 m/s), well below the strength of

most standard tropical cyclones (usually Category 1 or greater) when they first begin to develop

an eye feature (CIMSS). In addition, cirriform clouds are observed converging into the system on

satellite loops of the storm’s early stages, also suggesting upper level convergence into the low

pressure centre. The combination of incoming cirrus clouds on satellite, the lack of easily

identifiable outflow from the convective mass in Figure 2 and the sudden development of an eye

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inside the storm’s convective bands suggests that the eye formed in response to the need to

redistribute excess latent heat beginning to build in the upper atmosphere. This also seems to be

confirmed by the observations of unusually strong subsidence noted by Menezes (2004) and

Pytharoulis et al. (2000) in their analysis of Hurricane Catarina and the 1996 medcane

respectively. In particular, Pytharoulis’s model output for the 1995 medcane indicates that

sinking vertical winds within the eye of the storm were nearly as fast as the modeled vertical

motion within one eyewall, providing a powerful precedent for strong, well maintained

subsidence within the eye as a result of latent heat accumulation above the storm (and difficulty

diverging said heat).

For Catarina, the inability to diverge latent heat and the limited effectiveness of eye

subsidence in redistributing this outflow probably served to limit storm intensity at first, as the

large subsidence demanded a large eye (relative to the storm’s small size, limited by the

circulation of mid and upper-level low pressure systems) and suppressed convection. The

unsteady state of this convection is readily displayed on satellite loops of Catarina in its early

stages (not shown), which indicate that the cyclone underwent rapid convective replacement

cycles, with strong convection on one side of the storm often leading to greater convergence and

rapid suppression of convection on the opposite side of the storm.

Therefore, the scenario of rapid eye development as a result of latent heat accumulation

within the upper level low centre appears highly plausible given the theory and evidence at hand.

As latent heat continued to build while upper troposphere heights remained depressed in the

storm’s early stages, cyclonic, somewhat convergent rotating winds must have pushed most

latent heat toward the storm center rather than away from it, as demonstrated in Figure 3(b). In

other words, latent heat introduced above the storm was largely forced to converge, which in

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theory would have been accompanied by a localized pressure increase above the center of the

storm (the center of convergence) and strong sinking motion. Similar rapid eye development has

also been reported in most other midlatitude tropical-like cyclones, including medcanes,

Hurricane Vince, and Hurricane Huron, and these are likely due similar thermodynamic-dynamic

interactions at the upper levels.

Standard Hurricane Catarina (initially) Figure 3: Primary circulation in (a) a standard tropical hurricane and (b) Hurricane Catarina

However, in order to confirm that latent heat flow indeed does conform to the theoretical

precepts and empirical observations presented above, more direct measurements of upper

atmospheric latent heat redistribution flow will be needed over these storms, which might be

accomplishable by extensive dropsonde measurements into open ocean midlatitude tropical-like

cyclones, frequent analysis by hurricane hunter planes, or higher resolution sounder wind data

from satellites or airplanes directly over the convective regime (and over which data is sparse in

the Figure 2 above, the only available dynamic flow image of this storm in its early stages). In

general, the limitations and observational shortcoming of the above interpretation may be

resolved by a better picture of the early-mature upper level environment above other cyclones

similar to Hurricane Catarina.

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3.1.2 Hurricane Catarina’s Mature Stage: The Concentrated Nature and Slow Expansion of the Storm’s Upper Level Latent Heat “Bubble”

Figure 4: Upper Level Satellite-Derived Winds over Hurricane Catarina, 1500 UTC 26 March, 2004

The very presence of a warm core seen by models and remote sensing data during the

storm’s mature stage also helps establish the nature of the latent heat accumulation above the

system. In the case of Catarina, the NCEP reanalysis model failed to determine correct

temperature conditions at the upper levels above the cyclone as observed by the AMSU satellite-

determined brightness (temperature) values over the storm, which are more believable than the

cool anomalies depicted by NCEP graphics given the latent heat-driven nature of the cyclone.

NCEP data indicated that the storm began its life with a warm core at 200 mb but entered its

mature stage with a cold anomaly at 200 mb (not shown). While this decrease in temperature

might indicate the weakening of the upper level trough and perhaps even the formation of a

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regional ridge-type anomaly over the cyclone center, the actual colder temperatures represented

in NCEP output graphics (which are not backed by the presence of nearby sounding data)

directly contradict the warmer brightness values observed in the storm, shown in Figures 5(a-d).

The brightness images do confirm that a distinct warm core, or bubble of warmer temperatures

due to the release and accumulation of latent heat, existed above the storm. Figures 5(a) and 5(b)

demonstrate that temperatures rapidly increased immediately above the cyclone within a five

hour period, and the majority of this heat appears to be constrained and trapped over the center of

the surface cyclone, corresponding to strong subsidence within the eye and limited divergence

around it. In addition, in Figure 5(c), a brightness image taken a few hours later, the warm core

appears strongly concentrated in a small circle at 350 mb, with a much larger but still

concentrated bubble of warm air right above the center of the system at the 200 mb level (near

the tropopause) seen in Figure 5(d) taken at the same time. Although this warm air has been

allowed to diffuse somewhat on the west side of the cyclone (where the divergence outflow—

discussed below—was strongest), the graphic also indicates that a large amount of latent heat

(significantly more than was seen at the 350 mb level) had been forced to accumulate right above

the storm, indicating that the eye likely played a crucial role in redistributing heat, especially

given the relative strength of the system as a category 1 hurricane (where such great

accumulations of heat relative to slightly lower levels are rare).

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Figure 5: AMSU Brightness graphics illustrating the warm core of Catarina at (a) 200 mb during Catarina’s early mature stage, (b) 200 mb during Catarina’s mature stage, and (c) 350 mb and (b) 200 mb during Catarina’s late mature stage

Comparison with other tropical cyclones of similar and greater intensity using brightness

images further illustrates the abnormality of the high accumulations of latent heat at the upper

levels above Hurricane Catarina. For example, powerful Hurricane Floyd (during peak intensity

at 0019 GMT on 14 September, 1999) possessed a weak gradient between upper and lower

levels, with brightness values in the upper troposphere yielding -26.2°C at 350 mb and -37°C at

250 mb, a relatively small 10.8°C lapse between the two levels (Bevens, 2005). Because

Hurricane Floyd had strong convergent winds, much stronger than Hurricane Catarina, it also

Simmons 22

transported large quantities of latent heat to the upper atmosphere, which in turn provided an

upper level warm core pronounced enough to significantly reduce the negative lapse rate

between the 350 mb and 200 mb levels.

A small hurricane in the same tropical environment might have a greater gradient

(Category 1 Hurricane Stan in 2005 -- 14°C) due to less latent heat release (and thus cooler

temperatures at the 200 mb level and a larger lapse rate) (Bevens, 2005). On 27 March, 2004,

after some divergent outflow had already been established, Catarina possessed a core

temperature of -32°C at 350 mb and -44°C at 200 mb, having lower temperatures at both levels

(due partly to the nature of the cut off low and higher latitudinal environment) but also

possessing a heat gradient (12°C) similar to the Category 4 Hurricane Floyd (and on 26 March at

2035 GMT, seen in Figure 2(b) and before divergent flow had become prominent, Catarina had a

heat gradient of 11°C). This indicates that an excessive amount of heat relative to the storm’s

intensity was likely trapped above the cyclone as a confined bubble (due to its relatively

inefficient divergence) and that subsidence likely played an incredibly important role in

maintaining the system during this stage of the storm’s development.

3.1.3 Hurricane Catarina’s Late Mature Stage: The Creation of Secondary Circulation Channels to Aid the Upper Level Divergence of Latent Heat

However, while strong localized subsidence may displace some of the converging latent

heat, it is unable to redistribute all of it without completely suppressing the storm. Thus, the

greater and greater buildup of heat leads to an upward and inevitably outward expansion of air

that eventually fills in the upper tropospheric low minimum right above the storm and even

creates a localized upper tropospheric high (within a broader synoptic-scale tropospheric low).

This feature is implied by the brightness profile in Hurricane Catarina, associated with the warm

pool immediately above the storm, and was directly noted by Sousounis et al. (2001) in

Simmons 23

Hurricane Huron, where latent heat was observed causing warming in the upper levels,

increasing height contours, decreasing upper level temperature gradients, and weaker winds and

cyclonic motion at the upper levels near the storm (which in turn aided in the storm’s dissipation)

(Sousounis et al., 2003). Hurricane Catarina, however, achieved a more efficient way of

diverging excess latent heat which allowed the storm to maintain its strong uplift and not fall

under the influence of subsynoptic scale subsidence from the upper levels as seen in Hurricane

Huron. Figure 4, which depicts synoptic scale winds associated with Catarina on 12Z on 25

Figure 6: 250 mb pressure heights above Catarina at (a) 00Z 26 March, 2004, (b) 00Z 27 March, 2004, and (c) 00Z 28 March, 2004. March, 2004, indicates (as Bevens (2005) confirms) that Catarina obtained divergent flow at the

upper levels extending outward from the center of the storm, aided by a small cyclonic

circulation anomaly along a jet streak associated at the edge of a tear off ridge building eastward

into the downstream jet. Also, the satellite-derived winds demonstrate that the former upper level

low’s cyclonic rotation, formerly directly over the storm, had become much broader and weaker,

suggesting rising pressure associated with Catarina’s distribution of latent heat to the upper

levels. Accordingly, Figures 6(a) through 6(c), which show the NCEP synoptic-scale evolution

of pressure heights, indicate that height contours slowly increased above the storm and the upper

level low weakened significantly from 00Z 26 March to 00Z 28 March, in itself indicating an

environment more favorable for divergent outflow.

Simmons 24

In addition, Figure 6(c) reveals that by the late mature stage of the storm, the low

pressure minimum, sustained by the presence of cyclonic curvature (and associated vorticity) in a

persistent subtropical jet, had shifted to the north of the surface cyclone, and a weak anticyclone

provided by a ridge in the polar jet formed to the south of the storm. The lack of topography over

the ocean allowed relatively undisturbed airflow between these two anomalies and the storm

itself. Figure 7 below, depicting the synoptic situation six hours before Figure 6(c) above,

confirms the presence of a trough in the subtropical jet and slight ridge in the polar jet, which

induced these two circulation anomalies. Most important for the sake of Catarina’s strength, both

circulations appear to take the outflow latent heat away from the storm and transport it toward

the parent jet, providing adequate divergence channels for Catarina to thrive and preventing

widespread subsidence and the associated suppression of convective clouds in the storm center.

Several remote sensing methods demonstrate the presence and effects of these circulation

anomalies. Satellite loops of the storm show high level cirrus clouds moving north of the storm

and circulating around this cyclonic circulation anomaly, suggesting the transport of air and

latent heat out of the storm along the cyclonic circulation channel. Also, the 200 mb brightness

sounder profile depicted in Figure 5(d), taken just as the two circulation anomalies began to take

shape, also seems to show warmer latent streaming and diffusing to the northwest of the storm

along the low pressure vorticity anomaly. In addition, the importance of the strong outflow

channel to the south of Catarina is clearly apparent in the 350 mb brightness image (Figure 5(c)),

which shows a -33°C isotherm curving to the south along the outflow streamline seen in Figure

7, and this image also shows this isotherm stretching to the northwest along the cyclonic vorticity

anomaly.

Simmons 25

Figure 7: Upper Level Winds above Hurricane Catarina at 1800 UTC 27 March, 2004

The next 200 mb brightness sounding taken a few hours later, presented in Figures 8(a-b),

also suggest the importance of these circulation anomalies. Specifically, the lapse rate between

the 350 mb (Figure 8(a)) and 200 mb levels (Figure 8(b)) increased, while the 200 mb

temperature decreased notably in temperature above the cyclone’s core to -44.5ºC and more

dramatically just outside the storm center. This decrease contrasts with the trend seen in Figures

5(a-d), which indicates slowly increasing brightness values above the storm at the 200 mb level

as it evolved during its mature stage. The sudden drop in brightness values around the cyclone

and pronounced decrease in temperatures immediately above the storm, at a time when the

system maintained the strongest winds (according to scatterometer satellite estimates) and thus

brought in the greatest amount of moisture and latent heat, suggests that these circulation

anomalies were suitably effective in removing latent heat from above the storm and providing

greater Carnot Engine-like efficiency (CIMSS).

Simmons 26

Figure 8: AMSU brightness temperatures above Catarina at (a) 350 mb and (b) 200 mb

Figure 7 also illustrates the presence of an important outflow anomaly associated with the

storm itself, and this small perturbation in the mean flow appears more significant in the

redistribution of latent heat than the weak ridge anomaly in the jet. Wind bars in Figure 7 coming

into the cyclone from its east side appear to curve anticyclonically toward the southern part of

the storm, indicating negative vorticity in this region and suggesting localized higher pressure

heights. Indeed, this side of Catarina is associated with the strongest convection, located along an

inflow of low and mid-level moisture following the subtropical jet east of the storm, and this

observed feature may reflect MTP observations taken over North Atlantic Hurricane Emily,

which indicated that the highest tropopause heights were located in a small pinnacle directly

above the storm’s strongest (highest-topped) convective clouds. This outflow channel into the

polar jet appears to be enhanced by a small, shortwave trough framing the polar jet ridge.

Additionally, the polar jet ridge itself was likely enhanced by outflow from the storm,

which, by causing rising heights to its north and south to the point of shifting the upper level low

PV anomaly northward, would have been able to increase pressure heights to the south as well to

build the ridge (and theoretically the strength of the ridge outflow channel). McTaggart-Cowan

et al. (2005) claims that this ridging in the polar jet acts as one of the most important sinks for

latent heat above the storm, and this seems to be illustrated on NCEP pressure maps indicating

Simmons 27

the disconnect between the typical blocking pattern of the midlevel flow (500 mb) and upper

tropopheric rex block pattern (300 mb and above). This building of outflow into the ridge might

also be indicated by high 200 mb brightness temperatures curving slightly to the south, although

most of the heat appears to be moving along the stronger cyclonic circulation to the north-

northwest. However, the wind flow pattern illustrated in Figure 7 does not show a pronounced

anticyclonic circulation channel in the ridge that would provide a direct outflow channel out of

the storm. In fact, Figure 7 indicates that the main outflow channel from the south side of the

storm flows from east to west, in the opposite direction of any weak anticyclonic flow that might

lie to the south, and that the interaction between the anticyclonic spin over Catarina’s strongest

convective cells and the shortwave trough to the west is more significant in transporting this heat

into the jet. Similarly, satellite loops show upper level clouds moving out of the storm along this

outflow channel immediately to the south of the storm.

Simmons 28

Figure 9: NCEP reanalysis 200 mb pressure heights at (a) 12Z 25 March, 2004, (b) 12Z 26 March, 2004, (c) 12Z 27 March, 2004, and (d) 12Z 27 March, 2004

The NCEP analysis (Figures 6(a) and 6(b)) similarly shows fluctuations in this ridge

pattern over the course of the cyclone’s mature stage (instead of a slowly strengthening pattern).

NCEP provides the most substantial polar jet ridge at landfall when Catarina was strongest (and

latent heat fluxes through convergence and condensation would also have been strongest) and

closest to sounding data over land, which suggests that height building in the polar jet ridge was

associated with that over Catarina. However, as Figures 9 (a-d) illustrate, the ridge changes on

NCEP reanalysis graphics from pronounced during the storm’s early stages to nearly zonal to

pronounced again during Catarina’s late mature stage, suggesting that the ridge at least partly

derives from larger scale wavelength changes in the jet. However, because NCEP is unable to

resolve the latent heat bubble over the cyclone and the effects that it would have on the mean

flow, it cannot be relied upon as an adequate source of information on the fluctuations in the

polar ridge over the open ocean (where only estimations are provided) and thus is inconclusive in

determining a slow building of the ridge by latent heat as suggested by McTaggert-Cowan et al.

(2005). The satellite-derived wind graphics taken from 25 March to 27 March (Figures 2, 4, and

7) seem to indicate a building ridge throughout the storm’s life, although they are taken every 24

hours and are unable to provide a detailed illustration of fluctuations in the ridge. Thus, more

data is needed to understand exactly how much the anticyclonic ridge PV anomaly at the upper

levels is built by and interacts with the outflow regime of Catarina, but the airflow pattern

presented in Figure 7 clearly indicates that the storm’s interactions with various vorticity

anomalies produced by the subtropical and polar jets is crucial for its outflow divergence and

maintained intensity.

Simmons 29

However, brightness images 5(d) and 8(b), taken only a few hours after each other, also

clearly indicate that the strong latent heat anomaly continued to be present over the storm center.

Thus, these outflow circulations associated with the trough and ridge, while better in sustaining

the system than a total lack of divergent outflow, are limited in their effectiveness and are

certainly less efficient than tropical cyclones. Satellite and TRMM imagery (not shown) also

suggests that the eye, continuously expanding and contracting with processes similar to eyewall

replacement in large-scale hurricanes, persisted to play an important role in redistributing heat.

In addition, increased vertical shear associated with this double circulation setup might also

provide some limitations to storm strength, although the most significant shear between the

upper and mid troposphere continued to exist outside of center of the storm and the somewhat

shear-resistant bubble of latent heat above it. Therefore, despite their obvious limitations, the

relatively weak cyclonic and anticyclonic circulation anomalies located outside of Catarina

clearly played an important role in redistributing latent heat and allowing Catarina to intensify to

levels previously unseen in closely observed midlatitude tropical-like cyclones. This

interpretation, in turn, provides forecasters with an important impetus to track the latent heat

warm core and nearby circulation anomalies associated with storms like Catarina to determine

their relative efficiency and strengthening potential.

3.2 Hurricane Vince

North Atlantic Hurricane Vince (2005) provides another example of a midlatitude

tropical-like cyclone over the open ocean, and it also exhibits an upper level circulation pattern

associated with latent heat changes similar to that seen in Hurricane Catarina. Although not

officially dubbed as a midlatitude tropical-like cyclone by other authors as of yet, like most of

storms of this variety Vince developed underneath an upper-level low and maintained a

Simmons 30

subsynoptic-scale structure much smaller than the average size seen in standard tropical

hurricanes. In addition, it evolved into a hurricane very quickly (growing to become one of the

fastest forming hurricanes on record) and also sustained an eye feature when winds were under

hurricane force (National Hurricane Center Advisory Archive). Also, Vince formed over sea

surface temperatures of 23°C, deemed too cool for most subtropical cyclones by Bosart et al

(2003) in their analysis of subtropical storm development occurring in 2001 (National Hurricane

Center Advisory Archive; Wetter, 2005). An NCEP reanalysis performed on the storm reveals

that the parent trough associated with Vince originated over Greenland and pushed south and

east across the ocean, visible in Figures 10(a) and 10(b) as it moved off the coast of Morocco

prior to Vince’s

Figure 10: 200 mb pressure heights associated with Vince’s parent cut off low, during Hurricane Vince’s developing stage at (a) 00Z 8 October, 2005, and (b) 06Z 8 October, 2005 formation. Like Catarina, any cold air filtering into the trough had followed a long fetch over the

ocean and, therefore, had likely been significantly modified by the ocean waters, although the

steep pressure height gradient seen at 200 mb in Figure 10(a) and 10(b) indicates that the air

inside the trough was still very cold compared to its environment, providing a strong thermal

imbalance characteristic of midlatitude tropical-like cyclogenesis. As the trough cut off,

Simmons 31

decreased strong advection reduced shear and provided an environment favorable for tropical

cyclone-like convection. Although Hurricane Vince in many ways resembles Hurricane Catarina,

the other primary example of open ocean midlatitude tropical-like storm development, unlike

Catarina Vince did not develop within a trough that had interacted with mountain ranges in the

storm’s initial stages.

3.2.1 Vince’s Secondary Circulation Anomaly

Vince’s mature stage, however, followed a pattern very similar to that associated with

Hurricane Catarina, with an initial buildup of latent heat above the storm causing an eye to

develop quickly, and continued heat accumulation over the cyclone promoted the formation of at

least one important secondary circulation anomaly. While NCEP did not resolve the independent

pressure minima associated with observed vorticity anomalies to the north and south of the storm

as it had in Hurricane Catarina, satellite loops over Vince do confirm the presence of a cyclonic

vorticity channel just to the south of the cyclone as it began to strengthen (Wetter, 2005). This

feature is visible in Figure 11(a) as a cyclonic spiral of high level cirriform clouds exiting the

south side of the storm and curving cyclonically into the jet, which confirms the transport of

latent heat and upper level moisture out of the top of the storm.

While Vince developed under a series of vertically stacked upper level lows, the same

processes seen in Catarina, an increase in heights above the center of the storm due to latent heat

release and a subsequent shifting of the true pressure minimum toward the south appears to be a

viable explanation for the presence of the cyclonic vorticity channel. Similarly, the movement of

this vorticity anomaly (smaller than that associated with Catarina) away from the storm as Vince

evolved with time probably also demonstrates the continued raising of heights above the warm

core system due to increasing latent heat inflow at the upper levels (indicating that the circulation

Simmons 32

anomaly, like in Catarina, provided much-needed divergence of this energy but was not perfect

in preventing continued latent heat accumulation above the storm at the upper levels). However,

despite the movement of this feature further and further away from the storm, satellite imagery

(such as in Figure 11(b)) indicates that upper level clouds continued to stream from Vince into

the small vorticity anomaly for an extended period of time (although perhaps with less efficiency

than before).

Figure 11: IR Satellite Image of Hurricane Vince demonstrating a cyclonic vorticity channel (a) during the mature stage (b) during the late mature stage

Unlike in Hurricane Catarina, a similar outflow channel near an anticyclonic vorticity

anomaly was not readily apparent by observing cirrus cloud motion on satellite loops. However,

the existence of a pronounced ridge off the coast of Spain, visible on Figure 10(a), might indicate

the presence of such an anomaly during the storm’s early stages (but before the storm’s latent

heat could readily interact with such a perturbation). More data, including multilayer satellite

observed winds with high temporal resolution, would be helpful in determining the relationship

between Vince and the polar ridge and subtropical trough. However, standard IR satellite

imagery does confirm that a strong cyclonic vorticity outflow channel formed to the south of the

storm along the subtropical jet, and that this anomaly likely served as the storm’s primary

outflow channel. In addition, satellite scatterometer estimates confirm that wind speeds reached

Simmons 33

maximum intensity, quickly reaching Category 1 strength, after the development of upper level

cyclonic spin to the south of the storm (NRL). Thus, like in Hurricane Catarina, the presence of a

secondary circulation anomaly appears crucial for storm strengthening during the system’s

mature stage.

3.2.2 Shearing associated with a Jet Stream Kicker Prematurely Destroys Hurricane Vince

Figure 12: 200 mb level graphics at (a) 00Z 10 October, 2005, (b) 06Z 10 October, 2005, and (c) 12Z 10 October, 2005.

A more extensive analysis of Vince following a lifecycle similar in temporal span to that

of Catarina would have provided further evidence of the importance of vorticity anomalies in

determining the strength and formation processes open-ocean mid latitude tropical-like cyclones.

The fact that Hurricane Vince (like Catarina) quickly maintained Saffir-Simpson Category 1

force winds when other medcanes struggled to reach such strengths may be a clue to their

relative effectiveness in transporting latent heat away from the storm. However, Vince only

survived for a short time period of time, retaining its centralized hurricane structure and strength

for little more than a day before succumbing to shear (National Hurricane Center Advisory

Archive). Figure 12, representing the 200 mb trough pattern on 10 October, 2005 during the

storm’s dissipation, shows this shear in the form of an approaching kicker trough.

Simmons 34

Correspondingly, on satellite this kicker appears as an extensive cloud band associated with a

cold front at the surface (visible in the upper left corner of Figure 11(b)). The increasing shear

accompanying this trough worked to destroy the upper level pattern (and associated vorticity

anomalies) along with the centralized convective state of the midlatitude tropical-like cyclone

(National Hurricane Center Advisory Archive).

Thus, having rapidly succumbed to shear associated with the polar jet, Vince provides an

important example of the long-term unsteady state of the jet stream in the midlatitudes. The very

meridional flow pattern that creates deep troughs, cut-off lows, and associated medcanes can, if

persistent, lead to the eventual absorption and dissolution of cut-off lows and their associated

latent heat-driven storms. In this case it appears that the polar jet first works for Vince’s

development by staying ridged or zonal to the north (decreasing shear), but within a few hours

this wind stream brought down a deep, fast-moving trough that destroyed the storm through the

introduction of strong wind shear. While the upper level low pattern associated with Vince was

stable for several days as it moved south from Greenland and over the open ocean in the lower

midlatitudes, Vince formed (perhaps because of deepening due to continental air inflow from

Europe into the subtropical jet) several days after the original trough had become semi-separated

from the jet stream. Thus, ample time passed for even a slow-changing polar jet to produce

another deep trough to serve as a kicker for Vince. This scenario corresponds generally with

observations provided by Bosart indicating that the number of storm systems (in the form of

shortwave or medium-wavelength troughs) moving along the jet also likely determines whether

or not subtropical storms develop.

Hurricane Catarina did not come under the influence of kickers during its almost week

long evolution, and the polar jet remained stably zonal or ridged for nearly a week as Catarina

Simmons 35

evolved. Such a stable jet pattern is extremely rare in this region, according to Bevens (2005),

and had normal autumnal meridional oscillations remained prevalent in the jet over Patagonia,

aided by lee troughing provided by the Andes Mountains, Catarina would have likely lived a

much shorter life or might not have developed at all. Thus, while the open ocean environment

provides free interactions within the trough that allow the storm to reach greater strengths (due to

greater divergence of latent heat from Vince and Catarina’s centers), it also provides an unsteady

situation in which an oscillating polar trough can easily and rapidly destroy developing

midlatitude tropical-like cyclones.

4 Midlatitude Tropical-Like Cyclones that Interact with Topographical Features: The Lack of Latent Heat Divergence While Catarina and Vince provided important examples of midlatitude tropical-like

cyclones evolving over the open ocean with limited interaction with topographical features,

medcanes offer an example of the same phenomenon forming in a completely different

environment. One obvious difference between the two types of storms is that Catarina, with its

oceanic trough and long fetch for cold air advecting over the sea, was perhaps more closely

related to standard subtropical storms and Hurricane Vince (October 2005). Medcanes, on the

other hand, are associated with a stronger initial land-water contrast due to the advection of cold

continental air over the warm sea, perhaps positioned more closely on the cyclone spectrum to

convective polar lows. For the purposes of this study, medcanes offer an example of storms that

are heavily influenced by the direct or indirect effects of topography throughout their lives.

Fortunately, medcanes form in an environment surrounded by more weather stations than

are available near the Atlantic Ocean off Brazil, and NCEP data proves to be more helpful in

resolving certain features of medcanes’ upper level environments. As already established by

Simmons 36

Lagouvardos et al. (1999), Meneguzzo et al. (2000), and Kuo et al. (2002), medcanes are largely

driven by latent heat fluxes and thus this heat, like in Catarina and Hurricane Huron, tends to

accumulate at the upper levels in the form of a strong warm core. One way of tracking the

location and strength of such a latent heat perturbation above these storms is by analyzing

tropopause or near-tropopause level pressure or temperature data available from soundings and

NCEP model estimates that are available.

While NCEP data failed to confirm the presence of a strong perturbation of heat above

Catarina as seen on the brightness images, reanalysis of the medcane phenomenon, including the

1983 medcane and the early-mid October 1996 medcane, proves useful in demonstrating a strong

high pressure region and warm anomaly located directly or nearly directly over the storm center,

similar in its strongly concentrated nature to that observed on brightness images over the mature

Hurricane Catarina. The 1983 and 1996 medcanes were selected for examination because they

both passed extremely close to land and thus were more likely to come near atmospheric

sounding stations that the NCEP reanalysis program could process.

The 1982 medcane, which also closely followed the Italian and Greek coasts, was also

analyzed, and at 00Z on 17 October (not shown) a warm anomaly was clearly present over the

storm in NCEP graphics demonstrating atmospheric temperature above the storm. However,

most other NCEP upper level temperature readings from this particular event remained

inconclusive or failed to show a warm core, despite the obvious strength of the system and its

latent heat fluxes (which were established by Kuo et al. (2002) to be predominant due to a warm

core feature observed in the lower levels toward the beginning of storm evolution). According to

Kuo et al. (2002) and Reed et al. (2001), this might be due to erroneous or inadequately resolved

data, most likely associated with readings over North Africa, which could easily be responsible

Simmons 37

for a number of significantly inaccurate model runs performed in past studies of this storm. The

lack of a strong latent heat core might also be due to the presence of a deep upper level low and

subtropical jet-linked cyclonic circulation to the south of the storm over Tunisia, which might

have allowed a greater divergence of latent heat away from the storm center as in hurricanes

Catarina and Vince. Due to the fact that all other observed medcanes form directly underneath

the parent upper level low, and this storm formed within the occluded zone but just northeast of

the upper level low center (and this perhaps reflects the erroneous data over northern Africa), it

was not analyzed closely in this study.

4.1 The NCEP Evolution of the Upper Level Warm Core of the Early-Mid October 1996 medcane 4.1.1 200 mb Pressure and Temperature Characteristics of the Medcane

Figure 13: 200 mb temperatures during (a) early mature stage of the early-mid October 1996 medcane and (b) late mature stage of the same medcane

One of the best examples of medcane formation surrounded by topographical features is

provided by the early-mid October 1996 medcane, the synoptic evolution of which is discussed

in detail by Reale et al. (2001). Figures 13(a) and 13(b), which depict NCEP reanalysis

temperatures at 200 mb (250-150 mb are associated with the strongest part of the tropospheric

warm core of these systems) explicitly show an upper level latent heat anomaly located over the

Simmons 38

1996 cyclone, and they also suggest the strong correlation between an upper level warm core and

the surface storm center (which are located directly underneath the region of maximum 200 mb

temperatures). In addition, the same two figures demonstrate that this latent heat remained

concentrated directly above the storm during the 1996 medcane’s initial stages (Figure 13(a))

before beginning to spread out within the trough as the storm approached its early mature stage

(Figure 13(b)). However, there is not any evidence of circulation anomalies, either on satellite or

in NCEP wind data (not shown), that might have served to take heat away from the storm. Thus,

heat filling into the upper level low likely remained concentrated to the region of the storm and

was only able to spread through its own expansion of pressure heights within the upper level low

center. This gradual increasing of pressure heights in this storm is similarly illustrated in Figure

14 (a-c) and discussed extensively by Reale et al. (2001). Thus, instead of maintaining a stable

synoptic-scale trough pattern and secondary circulation anomalies like Hurricane Catarina, the

1996 medcane’s upper level low appears to simply fill in with the introduction of latent heat.

Figure 14: 200 mb upper level pressure heights above the early-mid October 1996 medcane at (a) 12Z 8 October, 1996, (b) 00Z 9 October, 1996, and (c) 12Z 9 October, 1996.

Thus, figures 13(a) and 13(b), when paired with Figures 14 (a-c), reveal that this latent

heat bubble above the 1996 medcane corresponded with a pressure height rise within the parent

trough. The NCEP data provides no evidence of a small tropopause fold over the subsidence in

Simmons 39

the eye; one could exist unresolved by the NCEP imagery, although due to the enhanced

convergence of latent heat into the eye and the less efficient nature of the eye in redistributing

this heat (likely leading to upward and outward swelling of a warm anomaly), this tropopause-

level feature might not be observed or as easily apparent in the storm’s early mature stages. In

some other images (not shown), the high temperature anomalies were shown to be skewed

toward nearby landmasses (as this is where the data that justifies the anomaly is derived from,

and thus NCEP is more likely to shift such centers toward land without resolving the details of

the storm’s center except on occasions when the storm itself moves over land).

A close analysis NCEP output depicting near-tropopause temperatures reveals additional

information about the nature of latent heat evolution during the typical lifecycle of a medcane.

Graph 1 display the maximum temperatures seen near the center of the 1996 cyclone throughout

its life, and it highlights an expected pattern of latent heat loss and gain at the upper levels

associated with landfall and redevelopment. The NCEP reanalysis run consistently resolved a

warm anomaly over the storm, which saw its greatest heat levels immediately above the storm

during its early development, the strength of which is likely related to both the presence of

building latent heat along with a warm stratospheric intrusion at this pressure level into the cold

upper-level low. However, this latent heat quickly subsided or spread away from the low center

after landfall on Sardinia, resulting in a sharp decrease in temperatures immediately above the

storm. Although the upper level trough had been weakened significantly by the gradual

introduction of heat to the atmosphere, the temperature gradient between the lower and upper

levels appeared strong enough to support the reintensificiation of the cyclone, and upper level

temperature maximum began increasing again while isotherms once more started to spread

outward as the medcane reached peak strength. The warm anomaly also further expanded within

Simmons 40

the trough and further decreased heights to the point that the trough and the associated medcanes

disintegrated after reaching the Aeolian Islands (due to loss of latent heat fluxes from the surface,

orographic effects from the mountainous islands, and convergence along the coasts). Upon

reaching Calabria, the storm’s upper level temperatures dropped rapidly again, likely a

demonstration of the subsiding and spreading of latent heat in the upper atmosphere associated

with the loss of incoming water vapor from the surface. At this point the pressure heights

associated with the upper level low over the cyclone deepened slightly, likely related with the

rapid subsidence and loss of heat in the upper levels (this was also observed over Catarina after

the hurricane made landfall), although the trough had been filled by latent heat to such an extent

that redevelopment in the Ionian Sea proved impossible.

200 mb Temperatures Above the Centre of the Early-Mid October, 1996 Medcane

220

220.5

221

221.5

222

222.5

223

223.5

224

224.5

225

18Z 7October, 1996

00Z 8October, 1996

06Z 8October, 1996

12Z 8October, 1996

18Z 8 October1996

00Z 9October, 1996

06Z 9October, 1996

12Z 9October, 1996

18Z 9 October1996

Date and Time

Tem

pera

ture

(deg

rees

Kel

vin)

Graph 1: NCEP-estimated 200 mb temperature above the early-mid October, 1996 medcane from formation to dissipation

Simmons 41

4.1.2 NCEP Reanalysis Using Tropopause-Level Pressure Data While NCEP-derived tropopause-level temperature maximum (not shown) appeared to

follow the center of the early-mid October 1996 medcane during its early and early mature

stages, NCEP then shows it bowing to the south over Sicily and Africa (away from the storm

center) upon reaching maturity between Sardinia and the Aeolian Islands. An analysis of

pressure heights and U-wind and V-wind NCEP data (not shown) does not indicate that a

circulation anomaly or other kind of wind served to draw this heat away from the cyclone center,

and thus the transport of the warm core away from the cyclone toward Sicily may again be

attributable to a lack of sounding data over the storm as it pushed away from Sardinia and back

over the open Mediterranean.

Figure 15: Pressure (Pa) at the tropopause level during the early-mid October 1996 medcane’s developing and early mature stages, at (a) 18Z 7 October, 1996, (b) 00Z 8 October, 1996, and (c) 06Z 8 October, 1996.

The model does, however, maintain the latent heat bubble in the form of a region of

increased pressure heights located over the center of the storm when a tropopause level pressure

analysis was used. Like in the 200 mb pressure analysis, the tropopause pressure height

maximum follows two major patterns—the raising of the upper level low heights (which

decreases the resolved temperature and pressure at the tropopause on a synoptic scale) and the

Simmons 42

increasing of pressure heights locally above the storm center. The spreading of latent heat within

the storm center is visible in Figures 15 (a-c) as the 27000 Pa isobar (decreasing in coverage in

previous images) expands rapidly to cover a much broader area as a 28000 Pa contour becomes

positioned directly above the storm center. In addition, the 27000 Pa isobar retains its broad

circumference around the storm as it moves onshore over Sardinia. Graph 2 indicates, as seen in

the 200 mb temperature table above, that the localized heights above the medcane collapsed in

the following period after it moved over Sardinia (likely associated with the collapse of latent

heat fluxes). After moving back over the sea, however, the pressure heights began to build, and

build consistently, until the storm made landfall on the Aeolian Islands and Calabria (at which

point the tropopause-level pressure dropped by several thousand pascals associated with the

sudden collapse of the latent heat bubble through the cessation of inflow and net subsidence).

Tropopause-Level Pressure Over the Centre of the Early-Mid October 1996 Medcane

22000

23000

24000

25000

26000

27000

28000

29000

18Z 7October,

1996

00Z 8October,

1996

06Z 8October,

1996

12Z 8October,

1996

18Z 8October,

1996

00Z 9October,

1996

06Z 9October,

1996

12Z 9October,

1996

18Z 9October,

1996

Date and Time

Trop

opau

se P

ress

ure

(Pa)

Simmons 43

Graph 2: NCEP-estimated tropopause-level pressure (Pa) above the early-mid October, 1996 medcane from formation to dissipation

The persistent increase in tropopause level pressure heights above the medcane as it

moved over the water, mirrored by the same tendency in the 200 mb temperature NCEP

reanalysis, contrasts markedly with the trend in brightness temperature at the same pressure level

over Catarina (which rose through the mature stage and then fell during the development of

circulation anomalies at the cyclone’s greatest intensity). This fact, accompanied by an increase

in pressure heights over the upper level low as the storm intensified and a lack of a circulation

anomaly visible on satellite and on the NCEP reanalysis pressure and wind fields, suggests that,

unlike Catarina, the 1996 medcane did not maintain an efficient method of divergence and

simply filled in the upper level low. Satellite images (not shown) also show that the storm’s eye

increased rapidly in size before reaching the Aeolian Islands as latent heat continued to be

transported into the cut off low, suggesting that the buildup of air molecules in the upper levels

was forcing even more subsidence in the eye. This process would have eventually led to

widespread sinking motion that might have aided in the dissipation of the storm, such as that

seen in the case of Hurricane Huron and the 1995 medcane (discussed below) (Sousounis et al.,

2001). This is also suggested by the fact that the storm did not regenerate after moving across the

relatively thin peninsula of Calabria, despite the fact that it had done so after making landfall in

Sardinia (and other medcanes, such as the 1983 storm, also regenerated after making landfall

given a continued thermal inbalance).

Therefore, it appears that the early-mid October 1996 cyclone survived as long as a

strong heat imbalance existed between the lower and upper atmosphere, and once the trough, the

cool pool, had been filled in, it lost its regeneration potential. In addition, NCEP data suggest that

latent heat concentration continued to build as the storm evolved, in contrast to the gradual

Simmons 44

increase in latent heat over Catarina’s center followed by a decrease in brightness values during

the strongest stage of the storm when the circulation anomalies aided divergence. Thus, unlike

Catarina, which maintained its trough structure and total surface-to-tropopause atmospheric

instability for an extended period of time by circulating heat toward the subtropical and polar

jets, the 1996 medcane simply strengthened until it had filled in its parent trough, after which it

began to gradually weaken. While the NCEP model is not as finely resolved to detect height

increases above the eyewall versus the eye (as in the MTP observations), nor can it capture the

location and concentrations of heat as finely as brightness imagery, the graphics do indicate that

the entire storm appears to be associated with a strong, broad subsynoptic rise in the tropopause

(unlike the concentrated rises above the strongest convective cells in hurricanes) associated with

a very concentrated pool of latent heat that continued to increase in strength throughout the

storm’s mature stage, indicating that the storm’s inefficiency in redistributing latent heat

significantly decreased its strengthening potential. This suggests that the depth of pressure

heights associated with the parent trough is directly proportional to storm longevity in the

medcane environment. Indeed, extremely deep troughs that take longer to fill with latent heat,

such as those in the 1982 and early-mid October 1996 storms, are associated with stronger,

longer lasting medcanes than weaker cutoff troughs.

4.2 Evidence from the Early October 1996 Medcane and the 1983 Medcane

Other medcanes were also analyzed with the NCEP reanalysis model as part of this study,

and they reveal a somewhat similar pattern. Unfortunately, the lifespan of the early October 1996

medcane was too short to provide any evidence of a change in 200 mb level or tropopause-level

temperature and pressure, but the notable increase in pressure heights at the 200 mb level during

Simmons 45

the cyclone’s developing stages suggests that the upper level low began to fill. In addition, this

particular upper level low was not as strong as many similar troughs associated with other

medcanes, and thus the latent heat released by the storm likely filled in the low quickly, not

allowing it to regenerate after the cyclone had passed over Calabria and largely dissipated. In

addition, the approach of another significant trough toward the Mediterranean (later to become

associated with the early-mid October 1996 medcane) also likely limited the storm’s potential for

further strengthening.

Figures 16: 200 mb pressure heights at (a) 00Z 4 October, 1996 and (b) 12Z 4 October, 1996

An analysis of the 1983 medcane also determined that the storm possessed a warm

anomaly that appears to be associated with the subsynoptic low, resolved as a bubble of 221.5ºK

air (surrounded by an isotherm) moving off the coast of Sardinia and persisting until it reached

the Genoan coast of Italy. In this case, the tropopause level temperature data mirror the

tropopause level pressure relatively consistently, and the temperature graphics (show in Figures

17 (a-f) below) shows a pattern very similar to other medcanes, with a spreading of latent heat at

the upper levels (Figures 17 (a-d)), collapse of said heat with landfall over Sardinia (Figure 17

(e)), and an increase in latent heat content at the upper levels again as the storm regenerates over

the sea (Figure 17(f)). In addition, NCEP reanalysis 200 mb pressure heights (not shown) were

observed to be increasing above the storm throughout the period, indicating that the medcane’s

Simmons 46

primary role was to reestablish the thermal disequilibrium between the surface and the

atmosphere before its dissipation.

Figure 17: Tropopause level temperatures at (a) 00Z 28 September, 1983, (b) 06Z 28 September, 1983, (c) 12Z 28 September, 1983, (d) 18Z 28 September, 1983, (e) 00Z 28 September, 1983, (f) 06Z 28 September, 1983.

4.3 Satellite Evidence from the 1995 Medcane: Increased Subsidence Over Time Associated With Latent Heat Buildup

The 1996 medcane provides another important, albeit indirect, example of the importance

of latent heat accumulation above the storm. Unfortunately, NCEP reanalysis proves unhelpful in

demonstrating this lifecycle in the 1995 cyclone due to its greater distance from land during its

evolution and the presence of a nearby synoptic-scale low pressure center. However, it is

Simmons 47

particularly helpful to look at this example because it is the only medcane to fully evolve to

maturity and then weaken over the open sea before making landfall.

Figure 18: the 1995 medcane on IR satellite before landfall and dissipation

NCEP data does confirm the large scale flow described by Pytharoulis et al. (2000) and

indicates that the cold trough’s upper level low was positioned directly over the parent storm

(series of figures to be placed here at a later date demonstrating this) and that it gradually

associated with increased heights (and presumably increased temperatures) as the storm evolved.

Satellite images (not shown) indicate that the storm saw explosive development within the

trough, but after 24 hours the heights associated with the trough began to fill, and Pytharoulis

noted that some weak anticyclonic upper level divergence was present over the storm, suggesting

the warm core of the system was expanding into the trough and increasing pressure heights

above the cyclone (Pytharoulis et al., 2000).

At the same time (12Z on 15 January), cumulonimbus cloud tops began to be surpressed,

causing a drop in IR cloud top temperatures from -50 to -60 degrees Celsius to -40 degrees

Celsius (Pytharoulis et al., 2000). By 16 January, the convection within the storm had weakened

considerably and the eye had expanded to take up the majority of the storm’s circulation, with

only outer band-like structures curling around the system and little evidence of a consistent

eyewall feature (seen in Figure 18). This pattern suggests that widespread subsidence was

occurring in the system after the pressure heights of the trough had been filled. Thus, like the

Simmons 48

1996 medcane, the lifecycle of the 1995 cyclone appeared strongly correlated with the deepening

and weakening of the upper level trough, as observed with the 1996 medcane and Hurricane

Huron (Sousounis et al., 2001). Without the divergent channels like those established above

Catarina and Vince through their warm cores’ interactions with surrounding jet streams, these

medcanes could not resist larger scale subsidence associated with the increasing pressure at the

upper levels and the raising of tropospheric-scale trough pressure heights.

5 Discussion: The Nature of Mediterranean Basin Topography in Limiting the Flow of Latent Heat

Figure 19: A generalized view of an open ocean midlatitude tropical-like cyclone One major limiting factor in providing these outflow channels is the nature of the

topography of the formation environment. Over the open ocean, the relatively low-friction,

unobstructed troposphere provides the possibility for interactions between the subtropical jet to

the north and polar jet to the south. This process, along with the associated secondary circulation

channels, has been observed in Hurricane Catarina (and to a lesser extent, Hurricane Vince) and

is illustrated in Figure. This may be because, in deep troughs moving over an open ocean, cold

Simmons 49

air is able to spread across the ocean surface rather easily, without mountains or other

topographic and terrain features (and their related friction) obstructing the flow of cold air (with

primary inhibitor of southward-moving cold air being the Coriolis force). Thus, when a

midlatitude tropical-like storm develops, the region of cold air within the associated oceanic

trough exists outside the regime of the cyclone itself and is able to spread to the south into

warmer air and, by the thermal wind relationship, increases wind speeds associated with the

subtropical jet. In addition, this freer spreading of colder air serves to produce a much broader

trough than the topography-constrained cut off lows of the Mediterranean, which essentially

provides more low density atmosphere for the latent heat generated by the storm to fill in at the

upper levels. Thus, while Catarina was filling in heights above the storm with its warm core, the

continued presence and slow spreading of cold air at the lower levels outside the storm allowed a

persistent subtropical jet trough to exist, which provided a substantially large vorticity anomaly

that took latent heat away from the storm centre.

Figure 20: A generalized view of a medcane and the limitations in latent heat transfer provided by the topography

Simmons 50

However, the Mediterranean basin is surrounded by mountains that limit these kinds of

interactions, as shown in Figure 19. High topography on the northern side of the Mediterranean

restricts the protrusion of cold airflow, and thus the projection of troughs, over the warm sea to

certain key areas where lower topography and mountain passes allow greater cold airflow.

However, these passes are small and most troughs, once they form and push away from these

topography breaks, become completely cut off along these ranges and are unable to interact with

their parent jet streams due to the cold air blocking north of the Alps keeping the strongest

baroclinic zone (and, by the thermal wind relationship, strongest jet winds) well to the north of

the medcane. To the south, the Atlas Mountains serve to block cold air extending from the

prominent trough-producing Rhone Valley from moving south, creating a concentrated, highly

unstable pool of cold air north of the Atlas that is limited in its ability to spread. Thus, in

general, as a cold air pool moves over the Mediterranean and drifts away from its parent

topography break, the entire system becomes cut off from the polar jet flow to the north and

subtropical jet flow to the south (associated with the Alps and Atlas Mountains), and the effects

of the mountains on airflow (cyclolytic effects) limit the ability for air to pass unopposed over

the mountains at the 200 mb level.

In addition, these mountains serve to contain the pool of cold air over the Mediterranean

by restricting its spreading ability to the north and south and concentrating the coldest air within

the regime of the medcane storm itself. While this kind of blocking is favorable for producing a

concentrated unstable environment that encourages medcane cyclogenesis, it also prevents cold

air from spreading far away from the cyclone to produce, by the thermal wind relationship, a

strong jet streak at a distance from the storm capable of producing a cyclonic secondary

circulation anomaly. It also limits the trough space and the size of the low density air pool at the

Simmons 51

upper levels, creating a smaller region for latent heat to fill and thus causing a faster increase in

pressure heights above the storm. Similarly, as this topography-constricted cold air pocket is

largely overtaken by a medcane or medcane-like feature, rapid fluxes from the surface, which

may be associated with a warm core structure from the near surface layers to the upper levels as

demonstrated by Kuo et al. (2002) in the 1982 medcane, serve to heat the concentrated cold air

pool throughout the troposphere and prevent a strong baroclinic contrast and associated jet streak

from forming along the edges of the cut off low at a later time. Thus, unable to establish

circulation anomalies with the polar jet or subtropical jet, medcanes are more dependent on the

thermal disequilibrium for continued strengthening or redevelopment.

5.1 The Limited Contribution of the Subtropical Jet in the Mediterranean Basin and Similar Regions

Also, the very nature of the subtropical jet as a low latitude southern jet stream appears to

be a limiting factor to enhancing the medcane environment. While the subtropical jet

occasionally forms the southern jet edge associated with medcane troughs, providing a potential

for interaction with these convective storms, the development of most medcanes at 40°N latitude

limits these storms’ ability to interact with this feature, which is often weak or at a substantially

lower latitude in the late summer and autumn. In a rare event, the January 1982 storm possessed

a prominent subtropical jet along the southern edge of its associated medcane trough which

likely interacted with the storm. In general, if a medcane pushes east of Tunisia it will be better

able to interact with the faster winds of a persistent subtropical jet trough, which might produce a

cyclonic circulation channel as heights rise above the parent storm. In the case of the 1982 storm,

assuming that (despite likely errors in the data set) the synoptic-scale pattern is correct and an

upper level low associated with a strong subtropical jet in the medcane’s parent trough did

persist over Tunisia, this upper level circulation would have likely served as a strong latent heat

Simmons 52

circulation channel for much of the storm’s life, and this perhaps explains why the 1982 medcane

maintained strong convection and centralized structure for a long period of time (nearly four

days) before dissipating. The weakening stage of this system was characterized by the sudden

disappearance of the upper level low over Tunisia, marking the disintegration of the parent

trough. As the trough’s general low pressure minimum region became smaller and co-located

with the increasing heights over the mature medcane, latent heat likely subsequently filled in

what was left of the trough (heating becomes more pronounced in NCEP at this stage) and the

increase in pressure and greater subsidence probably contributed to the dissolution of the

cyclone.

The September 1983 storm developed in a similarly favorable environment for

interaction with a subtropical trough over lowland northeast Africa; however, the storm occurred

too early in the autumn for any real enhancement from a strong subtropical jet streak to the

south. Thus, the associated cut off low remained isolated in its environment with no resolved

circulation anomalies contributed by a jet streak to the south. In addition, the 1995 medcane did

develop during the same time of year and possessed no observable interaction with a subtropical

jet. Therefore, the enhancement of the subtropical jet clearly cannot be relied upon in the

Mediterranean basin, although if present and not obstructed by the Atlas Mountains it should be

monitored for the possible Catarina (or potentially 1982)-like upper level circulation anomalies

that can take latent heat out of the medcane and produce a more efficient system.

5.2 The Limited Contribution of the Polar Jet in the Mediterranean Basin and Similar Regions

Interaction with the polar jet ridge was also important for maintaining Catarina’s

divergent outflow its southern eyewall and reducing the amount of shear present within the

synoptic-scale setup associated with the storm. The inability of medcanes to build a ridge in the

Simmons 53

polar jet also fails to provide this added protection as well as the anticyclonic outflow channel

associated with this ridge (or the ridge edge) and the storm. The Japanese Kanto midlatitude

typhoon of October 10-11, 2003 demonstrated a similar process, with the northward-moving

cyclone’s warm core approaching Honshu failing to provide a northward ridge in the larger scale

polar jet located just to the north of the Japanese mountains. The fact that most medcanes and the

Japanese typhoon did not build ridges in their associated polar jets and did not interact readily

with it (due to the high topography) provided limited outflow from the north side of the storm

and likely led to latent heat (associated with the weak winds of the broader trough environment)

building into the trough and increasing overall heights while decreasing its potential for

strengthening. The inability of the Japanese storm, upon the introduction of a likely warm

thermal anomaly at the upper levels, to create an upward perturbation in the polar jet when

coming within a few hundred miles of it (and moving over Japan, where sounding data and

NCEP would have picked up this bending in the isobars along the polar jet), indicates that the

strong separation provided by the mountains served to further concentrate the latent heat bubble

rather than help in its dispersal.

Figure 21: 200 mb pressure maps for the 1983 medcane at (a) 12Z 28 September, 1983 and (b) 18Z 29 September, 1983

Simmons 54

However, there are some limitations to this interpretation. Some polar trough interaction

might have occurred during the redevelopment of the 1983 medcane off the Sardinia coast,

during which the NCEP analysis (see Figure 21 (a-b)) shows that the storm was located directly

south of a substantial thermal ridge at a time when it was better exposed to the polar jet through

the low topography of southern France, which might have helped transport latent heat away from

the storm through an anticyclonic PV anomaly like that established in Catarina. This ridge also

helps provide a reduced shear environment as the storm moved through a region (west of

Corsica) prone to cold air intrusions and kickers that could have absorbed the storm’s parent cut-

off system and subsequently dissipated the medcane itself. In addition, like McTaggart-Cowan et

al. (2005), Sousousnis et al. (2001), and Bosart et al (2003) suggest for Catarina and subtropical

storms, the building of the ridge might be partly attributable to diverging outflow from the storm

and partly characterized by natural fluctuations in the polar jet. The location of an outflow

channel in this region might also account for this storm’s higher divergence profiles at the 300

mb level stretched toward the west side of the storm, seen in Twitchell et al.’s 1989 textbook

discussion of the 1983 medcane.

In general, though, interactions with the polar trough becomes highly unlikely in cut off

lows moving east through the Mediterranean basin due to the high terrain located to the north of

these lows. Similarly, contribution from the subtropical trough or subtropical trough-like feature

has not been observed most medcanes (NCEP reanalysis (not shown) reveals that a subtropical

jet streak is lacking in the January 1995 medcane environment (despite its winter environment

and location east of the Atlas)). Thus, with the Atlas Mountains (and seasonal and latitudinal

considerations) limiting interaction with the subtropical jet and the Alps, Pyrenees, and Balkan

mountain ranges limiting interaction with polar jet, upper level dynamic airflow over the

Simmons 55

Mediterranean mirroring that of Catarina appears virtually impossible in most instances, which

might account for Catarina’s relatively greater longevity and strength compared to observed

medcanes. If Catarina had not been adjacent to a strong subtropical jet that remained prominent

even through the end of the storm’s life, then the secondary cyclonic circulation channel would

not have been as strong or able to carry as much latent heat away from the system, and greater

upper level divergence and strengthening would not have been possible as the center of the

trough quickly filled with latent heat being transported to the upper atmosphere.

5.3 The Medcane Environment’s Protection from Kickers

While the topography of the Mediterranean basin provides a less-than-optimal

environment for redistributing latent heat through jet interactions, it also affords much needed

protection from rapid oscillations in the polar jet that often destroys open ocean storms like

Vince. The all-encompassing orography surrounding the Mediterranean Sea creates a sheltered

region which vastly reduces the risk of incoming kickers that produce an increase in shear over

the storm environment (which leads, as in most tropical and subtropical storms, to rapid

dissipation). In particular, Bosart et al. (2003) discusses the importance of shortwaves in

preventing subtropical systems from forming, and the lack of such disturbances would

undoubtedly be beneficial to the stability of midlatitude tropical-like cyclones and subtropical

storms alike. As indicated above, only a few large-scale mountain passes admit troughs into the

Mediterranean basin, and these cool pools become cut off from the main flow and isolated once

they drift south of regions providing greater orographic shelter. This orographic effect was

readily apparent in the NCEP reanalysis of every medcane observed in this study (and can be

seen in Figures 16(a) and 16(b)), visible first as the tearing off of pressure heights from the polar

jet as upper level troughs move east below the mountains and then the movement of these cold

Simmons 56

pools along the axis of the Alps. Thus, once deep troughs moving over the sea cut off and drift

into a region of greater mountain blocking, other strong troughs and kickers moving across

Europe are less likely to disturb them (providing a particularly stable environment). In some

cases, such as the early and early-mid October 1996 medcane situations, several relatively

unaltered cut off lows can be maintained over the Mediterranean simultaneously, which affords

the remote possibility of a simultaneous medcane ‘outbreak.’

Open ocean midlatitude tropical-like cyclones, on the other hand, while perhaps able to

achieve better latent heat divergence than similar cyclones forming in topographically sheltered

regions, are more vulnerable to the deleterious effects associated with the rapid oscillations of

the unobstructed nearby polar jet. As indicated before, the extremely stable blocking pattern

associated with Catarina over the South Atlantic is extremely rare, which likely accounts for the

fact that no other similar hurricane has been observed in this region (Bevens, 2005). Another

weak cyclone that formed in the South Atlantic earlier that year (January 2004) was not so

fortunate and was quickly dissipated as it came under the influence of greater baroclinic shear.

Similarly, Hurricane Vince, as has already been demonstrated, deteriorated rapidly due the

presence of a nearby kicker trough. Thus, open ocean midlatitude tropical-like cyclones are rarer

and more easily destroyed due to their close proximity to the polar jet and its relatively frequent,

shear-producing oscillations. Thus, while the orography of the Mediterranean reduces ability to

establish for divergent outflow channels associated with jet stream-invoked vorticity anomalies,

the blocking of these jets and their sometimes destructive oscillations allows the latent heat

storms themselves to redistribute heat within their associated cold core upper level low with little

outside disturbance.

6 Conclusions

Simmons 57

In closing, after an extensive analysis of the apparent evolution of the upper-level warm

core of both the orographic and nonorographic cases, this study determines that open ocean

midlatitude tropical-like cyclones readily reach hurricane intensity because they are more freely

able to interact with strong upper-level winds to the north and south. The circulations in turn

establish pronounced but weak vorticity anomalies that serve to circulate latent heat away from

an associated latent heat-driven cyclone. On the other hand, medcanes are strongly influenced by

the presence of nearby mountains, with the northern polar jet separated from the storm by the

Alps and Balkan Ranges and the subtropical jet (if present) often separated by the Atlas. Being

almost entirely cut off from stronger wind channels, external vorticity anomalies do not develop

and latent heat from medcanes appears to simply accumulate and fill in upper level pressure

heights above the storm without diverging in an efficient manner. Medcanes are thus determined

to be poor Carnot Engine storms due to their relative inability to diverge latent heat through free

interactions with jet streaks to the north and south. However, the reasonably sheltered nature of

the mountain-lined Mediterranean basin from kicker troughs, appearing in time series NCEP

graphics as the stability of cut off lows associated with prominent medcane examples, allow

medcanes to thrive relatively undisturbed by external shear, and this makes the Mediterranean

basin a particularly favorable environment for midlatitude tropical-like cyclone formation.

Further research, however, needs to be done to demonstrate the effects of the latent heat

anomaly above midlatitude tropical-like cyclones. In particular, the role of positive and negative

vorticity anomalies around midlatitude storms, and each circulation’s relative efficiency in

redistributing this heat, needs further exploration, and this can be accomplished with the

acquisition of more comprehensive upper level data. Sounder brightness images and high

resolution satellite-derived winds from the upper levels above future medcanes, along with

Simmons 58

hurricane hunter missions, dropsondes, and greater coverage in ship sounding data would prove

invaluable in providing greater information about midlatitude tropical-like cyclones over the

open sea or ocean. In addition, dense mesoscale sounding networks established in highly

medcane-prone regions, such as Malta, Sicily, Aeolia, Calabria, and Sardinia, would provide

important information about the nature of the warm core of future medcanes as they make

landfall. All of these solutions would likely serve to fill the current void in data that has limited

the scope of this and other research, and it would also provide a more comprehensive picture of

the complex interactions between latent heat, eye subsidence, pressure height fluctuations, and

topography over these storms for the purposes of better intensity forecasting.

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Image Credits Cover Photo: Courtesy of NASA Figure 1: Courtesy of CIMSS Figure 2: Courtesy of CIMSS Figure 3: Created by Author Figure 4: Courtesy of CIMSS Figure 5: Courtesy of CIMSS Figure 6: NCEP Reanalysis Data Figure 7: Courtesy of CIMSS (arrows drawn by author) Figure 8: Courtesy of CIMSS Figure 9: NCEP Reanalysis Data Figure 10: NCEP Reanalysis Data Figure 11: Courtesy of Wetter et al. Figure 12: NCEP Reanalysis Data Figure 13: NCEP Reanalysis Data Figure 14: NCEP Reanalysis Data Graph 1: Created by Author Figure 15: NCEP Reanalysis Data Graph 2: Created by Author Figure 16: NCEP Reanalysis Data Figure 17: NCEP Reanalysis Data Figure 18: Courtesy of Bevens (online) Figure 19: Created by Author Figure 20: Created by Author Figure 21: NCEP Reanalysis Data